This invention relates to vehicle detectors used to detect the presence or absence of a motor vehicle in an inductive loop embedded in a roadbed. More particularly, this invention relates to a vehicle detector with an audible Call signal indicator.
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location. Such detectors have been used at intersections, for example, to supply information used to control the operation of the traffic signal heads, and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings. A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent the inductive loop mounted on or in a roadbed. In such systems, a first oscillator, which typically operates in the range from about 10 to about 120 kHz is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a fixed number of loop cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the fixed number of loop cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call Signal indicating the presence of a vehicle in the loop.
Correspondingly, if the difference between a sample count and the reference count is greater than a second threshold amount, this condition indicates that a vehicle which was formerly located in or near the loop has left the vicinity. When this condition occurs, a previously generated Call Signal is dropped.
The Call signals generated by a vehicle detector are used in a number of ways. Firstly, the Call signals are presented to an output terminal of the vehicle detector and forwarded to various types of traffic signal supervisory equipment for use in a variety of ways, depending on the system application. In addition, the Call signals are used locally to drive a visual indicator, typically a discrete light emitting diode (LED) or a multiple LED display or a liquid crystal display (LCD) to indicate the Call status of the vehicle detector, i.e. whether or not the vehicle detector is currently generating a Call signal.
Vehicle detectors with the Call signal generating capability described above are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, or in a parking stall, and numerous other applications. In all applications, a vehicle detector must be initially set up at the desired location by a qualified technician. Initial set up involves physically installing the detector unit into the housing provided (typically, a metal cabinet already present at the location), choosing certain detector parameters, such as loop frequency and detector sensitivity (typically by means of selector switches), and checking the operation of the detector in response to various vehicular conditions to verify that the detector has been installed correctly and is functioning in a correct fashion. Similarly, from time to time vehicle detectors must be checked for proper operation, and re-initialized, typically after a detector failure has been observed, either momentarily or permanently.
When initially setting up a vehicle detector, and when re-indicator to verify that the vehicle detector is operating in the proper manner by generating a Call signal when a vehicle enters the detector loop and dropping the Call signal after the detected vehicle leaves the detector loop. The technician performs this verification by observing the entry and departure of vehicles to and from the vicinity of the detector loop while simultaneously visually monitoring the state of the vehicle detector Call signal visible indicator. This visual observation is not always easily accomplished. In the presence of direct sunlight during the daytime, for example, the technician may have difficulty in visually determining whether or not an LED is illuminated. At night time, a technician cannot see an LCD unless back lighting is incorporated into the LCD display. More importantly, in a large number of vehicle detector installations the Call signal visible indicator device (LED or LCD) cannot easily be viewed by a technician while at the same time visually observing the flow of traffic through the lane associated to the vehicle detector, due to the physical placement of the vehicle detector unit relative to the detector loop. While this impediment can be overcome by sending two technicians to the vehicle detector site--one to observe the vehicle flow through the loop and the other to monitor the Call signal visible indicator--this is costly and does not address the possible visibility problems noted above regarding LEDs in direct sunlight and LCDs in darkness.